The present invention relates to communications, and more particularly to an ultra linear high frequency transconductor for use in communication networks.
Linearity is of utmost importance in the present day communication networks. In all wireless and wireline systems, it is very important to have low inter-channel interference and crosstalk. For example, in wireless networks it is important to maximize linearity in a channel to facilitate adapting to significant multipath propagation distortion. Linearization permits digital equalization techniques to resolve the multipath distortion.
Filters are a key component in any data communication system, providing anti-alias support, channel tuning, and channel matching. Since filters are in the communication path, filter linearity is crucial. There is strong motivation to realize filters on monolithic integrated circuits to reduce external component count, and hence, to reduce the cost for the overall communication system. There is also strong motivation to minimize power consumption since many communication applications are portable, such as cell phones, pagers, personal digital assistants (PDAs), etc. Filter techniques have achieved high linearity, but at the expense of high power, or alternatively, low power at the expense of poor linearity. More power efficient techniques are required.
A transconductor including a tunable transconductive device, a first amplifier circuit receiving a differential input signal, a common mode circuit, an input current source, first and second output current sources, and a second amplifier circuit that provides a differential output current based on the differential input signal. The differential input signal includes an input signal combined with a common mode signal. The first amplifier circuit applies the differential input signal to a differential input of the tunable transconductive device. The common mode circuit receives the differential input signal and a reference common mode signal, and provides a bias signal and a common mode feedback signal. The input current source is coupled to a two port output of the tunable transconductive device and sinks a constant input bias current. The output current sources are each coupled to the common mode circuit and provide output bias currents adjusted by the common mode feedback signal. The second amplifier circuit is coupled to the common mode circuit, the input current source, the tunable transconductive device and the output current sources, and maintains the two port output of the tunable transconductive device at a level of the bias signal and provides a differential output current based on the differential input signal. The transconductor may further be coupled to output capacitors to provide an output voltage that is proportional to the integral of the input signal.
In one embodiment, the tunable transconductive device includes first and second transconductive resistors and a tuning circuit in parallel with the first and second transconductive resistors. The tuning circuit has an adjust input for receiving an adjust signal. In a more particular embodiment, the tuning circuit comprises a Czarnul tuning circuit and series resistors. The series resistors are each coupled in series with a respective differential input of the Czarnul tuning circuit, where the series resistors and the Czarnul tuning circuit are coupled in parallel with the first and second transconductive resistors.
The first amplifier circuit may include first and second operational amplifiers, each including a non-inverting input that receives the common mode input signal, an inverting input and an output. The first amplifier circuit further includes first and second bipolar transistors, each having a collector coupled to a supply voltage, an emitter coupled to an inverting input of a respective one of the operational amplifiers, and a base coupled to an output of a respective one of the operational amplifiers.
The common mode circuit may include an input stage, a bias signal stage, and a differential amplifier stage. In one embodiment, the input stage includes a balanced Resistor-Capacitor (RC) circuit that receives the differential input signal and that derives the common mode signal. The differential amplifier stage receives the common mode signal from the input stage and the reference common mode signal and provides the common mode feedback signal. The bias signal stage receives the common mode signal from the input stage and generates the bias signal based on the common mode signal. The bias signal stage may further include a bias resistor receiving the common mode signal on one side and providing the bias signal on its other side, and first and second bias current sources coupled on either side of the bias resistor. Furthermore, the first and second bias current sources may each include a bandgap voltage source providing a precise control voltage, and a second bias resistor.
The first and second output current sources may each comprise MOSFETs coupled in a cascode configuration and operating in saturation. In a more particular embodiment, the MOSFETs are PMOS transistors, where a PMOS transistor at each differential output receives the common mode feedback signal from the common mode circuit. In this manner, the common mode circuit asserts the common mode feedback signal to minimize drift of the common mode signal of the circuit from the reference common mode signal.
In one embodiment, the second amplifier circuit includes first and second high gain amplifiers, each having a first input receiving the bias signal and a second input coupled to a respective output port of the tunable transconductive device. The second amplifier circuit may further include first and second current control devices, each having a current control path coupled to a respective one of the first and second output current sources and a control input coupled to an output of a respective one of the high gain amplifiers. In a more particular embodiment, the high gain amplifiers are operational amplifiers, each including an inverting input coupled to a respective output port of the tunable transconductive device and a non-inverting input that receives the bias signal. Also, the current control devices are MOSFETs, each having a current path coupled between a respective one of the output current sources and to an inverting input of a respective one of the operational amplifiers. In one embodiment, the operational amplifiers keep the output of a Czarnul tuning circuit at the level of the bias signal provided from the common mode circuit.
In an alternative embodiment, the transconductor includes a tunable resistive device, a voltage follower amplifier, a common mode circuit, an input current source, an output current source, a variable current source, and a high gain amplifier. The voltage follower amplifier receives a common mode input signal including an input signal combined with a common mode voltage, and provides the common mode input signal to the input of the tunable resistive device. The common mode circuit receives a reference common mode voltage and the common mode input signal and generates a bias voltage and a common mode feedback voltage. The input current source sinks a constant bias current from the tunable resistive device. The output current source receives the common mode feedback voltage and provides a DC output current based on the common mode feedback voltage The variable current source is coupled to the output current source and sinks current based on a control input. The high gain amplifier maintains the output of the resistive device at the bias voltage and controls the variable current source to sink output current based on the input signal.
The tunable resistive device may comprise a Czarnul tuning circuit and series resistors coupled in parallel with a transconductance resistor. The voltage follower amplifier may include an operational amplifier and a bipolar transistor coupled with negative feedback to assert the common mode input signal to the input of the tunable resistive device. The common mode circuit may include an input stage, a bias voltage stage, and a differential amplifier stage. The input stage may include a balanced RC circuit that receives the common mode input signal and that provides the common mode voltage. The differential amplifier stage receives the common mode voltage from the input stage and the reference common mode voltage and provides the common mode feedback signal. The bias voltage stage includes a bias resistor coupled between constant current sources, receives the common mode voltage on one side and provides the bias voltage on its other side. The output current source may include PMOS transistors coupled in cascode configuration and operating in saturation. The variable current source may comprise an NMOS transistor controlled by the high gain amplifier. The high gain amplifier may comprise an operational amplifier including an inverting input coupled to output of the tunable resistive device, a non-inverting input that receives the bias voltage and an output that drives the NMOS transistor to sink a current based on the input signal.
A filter circuit according to an embodiment of the present invention includes one or more common mode circuits, a plurality of inter-coupled transconductors, and a reference circuit that provides a reference common mode signal and an adjust input voltage for a tunable transconductive device of each transconductor. Each common mode circuit receives a differential input signal having a common mode signal and receives a reference common mode signal, and each provides a bias signal and a common mode feedback signal. The transconductors are each associated with one of the common mode circuits and each are configured in a similar manner as described above.